In borehole geophysics, a wide range of parametric borehole measurements can be made, including chemical and physical properties of the formation penetrated by the borehole, as well as properties of the borehole and material therein. Measurements are also made to determine the path of the borehole during drilling to steer the drilling operation or after drilling to plan details of the borehole. To measure parameters of interest as a function of depth within the borehole, a drill string can convey one or more logging-while-drilling (LWD) or measurement-while-drilling (MWD) sensors along the borehole so measurements can be made with the sensors while the borehole is being drilled.
As shown in FIG. 1A, a drill string 30 deploys in a borehole 12 from a drilling rig 20 and has a bottom hole assembly 40 disposed thereon. The rig 20 has draw works and other systems to control the drill string 30 as it advances and has pumps (not shown) that circulate drilling fluid or mud through the drill string 30. The bottom hole assembly 40 has an electronics section 50, a mud motor 60, and an instrument section 70. Drilling fluid flows from the drill string 30 and through the electronics section 50 to a rotor-stator element in the mud motor 60. Powered by the pumped fluid, the motor 60 imparts torque to the drill bit 34 to rotate the bit 34 and advance the borehole 12. The drilling fluid exits through the drill bit 34 and returns to the surface via the borehole annulus. The circulating drilling fluid removes drill bit cuttings from the borehole 12, controls pressure within the borehole 12, and cools the drill bit 34.
Surface equipment 22 having an uphole telemetry unit (not shown) can obtain sensor responses from one or more sensors in the assembly's instrument section 70. When combined with depth data, the sensor responses can form a log of one or more parameters of interest. Typically, the surface equipment 22 and electronics section 50 transfer data using telemetry systems known in the art, including mud pulse, acoustic, and electromagnetic systems.
Shown in more detail in FIG. 1B, the electronics section 50 couples to the drill string 30 with a connector 32. The electronic section 50 contains an electronics sonde 52 and allows for mud flow therethrough. The sonde 52 includes a downhole telemetry unit 58, a power supply 54, and various sensors 56. Connectors 42/44 couple the mud motor 60 to the electronics section 50, and the connector 42 has a telemetry terminus that electrically connects to elements in the sonde 52.
Mud flows from the drill string 30, through the electronic section 50, through the connectors 42/44, and to the mud motor 50, which has a rotor 64 and a stator 62. The downhole flowing drilling fluid rotates the rotor 64 within the stator 62. In turn, the rotor 64 connects by a flex shaft 66 to a drive shaft or mandrel 72 supported by bearings 68. As it rotates, the flex shaft 66 transmits power from the rotor 64 to the drive shaft 72.
Disposed below the mud motor 60, the instrument section 70 has one or more sensors 74 and electronics 76 to control the sensors 74. A power supply 78, such as a battery, can power the sensors 74 and electronics 76 if power is not supplied from sources above the mud motor 60. The drill bit (34; FIG. 1A) couples to a bit box 36, and the one or more sensors 74 are placed as near to the drill bit (34) as possible for better measurements. Sensor responses are transferred from the sensors 74 to the downhole telemetry unit 58 disposed above the mud motor 60. In turn, the sensor responses are telemetered uphole by the unit 58 to the surface, using mud pulse, electromagnetic, or acoustic telemetry.
Because the instrument section 70 is disposed in the bottom hole assembly 40 below the mud motor 60, the rotational nature of the mud motor 60 presents obstacles for connecting to the downhole sensors 74. As shown, the sensors 74 can be hard wired to the electronics section 50 using conductors 46 disposed within the rotating elements of the mud motor 60. In particular, the conductors 46 connect to the sensor 74 and electronics 76 at a lower terminus 48a and extend up through the drive shaft 72, flex shaft 66, and rotor 64. Eventually, the conductors 46 terminate at an upper terminus 48b within the mud motor connector 44. As with the lower terminus, this upper terminus 48b rotates as do the conductors 46.
Running conductors 46 through the flex shaft 66 creates difficulties with sealing and can be expensive to implement. FIG. 2 shows a prior art arrangement for hard wiring through a transmission section of a mud motor 60 between downhole components (sensors, power supply, electronics, etc.) and uphole components (processor, telemetry unit, etc.). The transmission section has a flex shaft 66 disposed in a housing and coupled between the rotor 64 and the drive shaft or mandrel 72. The flex shaft 66 connects the motor output from the rotor 64 to the drive shaft 72, which is supported by bearings 68. The flex shaft 66 has a reduced cross-section so it can flex laterally while maintaining longitudinal and torsional rigidity to transmit rotation from the mud motor 60 to the drill bit (not shown). A central bore 67 in the flex shaft 66 provides a clear space to accommodate the conductors 46.
The flex shaft 66 is elongated and has downhole and uphole adapters 69a-b disposed thereon. The shaft 66 and adapters 69a-b each define the bore 67 so the conductors 46 used for power and/or communications can pass through them. The adapters 69a-b typically shrink or press with an interference fit to the ends of the shaft 66.
Down flowing drilling fluid from the stator 62 and rotor 64 passes in the annular space around the shaft 66 and adapters 69a-b. The shrink fitting of the adapters 69a-b to the shaft 66 creates a fluid tight seal that prevents the drilling fluid from passing into the shaft's bore 67 at the adapters 69a-b, which could damage the conductors 46. A port 69c toward the downhole adapter 69a allows the drilling fluid to enter a central bore 73 of the drive shaft 72 so the fluid can be conveyed to the drill bit (not shown).
The flex shaft 66 has to be long enough to convert the orbital motion of the rotor 64 into purely rotational motion for the drive shaft 72 while being able to handle the required torque, stresses, and the like. Moreover, the flex shaft 66 has to be composed of a strong material having low stiffness in order to reduce bending stresses (for a given bending moment) and also to minimize the side loads placed on the surrounding radial bearings 68. For this reasons, the elongated flex shaft 66 is typically composed of titanium and can be as long as 4.5 to 5 feet. Thus, the shaft 66 can be quite expensive and complex to manufacture. Moreover, the end adaptors 69a-b shrink fit onto ends of the shaft 66 to create a fluid tight seal to keep drilling fluid out of the internal bore 67 in the shaft 66. Although the shrink fit of the adapters 69a-b avoids sealing issues, this arrangement can be expensive and complex to manufacture and assemble.
Other prior art mud motors have transmission sections with different configurations than disclosed above with reference to the fixed flex shaft. For example, FIGS. 3A-3C shows a prior art mud motor 60 that uses two drivelines 80 and 90 to facilitate a short bit-to-bend length. This mud motor 60 is similar to the 6.75-in. Oil Lube—SDB series mud motor available from Computalog Drilling Services, a predecessor to the assignee of the present application.
A top driveline 80 has a solid transmission shaft 82 that converts the rotor's orbital motion into pure rotational motion. One end of the solid transmission shaft 82 connects to the rotor 64 with an adapter 69b and a universal joint 84b, and the opposing the end of the drive shaft 82 connects to a bottom driveline 90 with a universal joint 84a. Because the solid transmission shaft 82 is exposed to drilling fluid inside the surrounding housing 65, both of the universal joints 84a-b are sealed with rubber seal boots to keep lubricating oil in and to keep drilling fluid out of the joints 84a-b. 
During operation, the drilling mud used to operate the positive displacement motor 60 flows from the stator 62 and the rotor 64 and into the annular space between the motor housing 65 and solid transmission shaft 82. From this upper section, all of the drilling fluid is then directed into an adapter's ports 86 that lead to the bottom driveline 90.
In the bottom driveline 90, the fluid flows into a central bore 93 of a piston mandrel 92b. The fluid then flows through a bore 93 of a second transmission shaft 92a and into a bore 73 of a bearing mandrel 72, from which the fluid can lead to a drill bit (not shown). Thus, this prior art motor 60 uses the bores 93 in the piston mandrel 92b and second transmission shaft 92a and the bore 73 in the bearing mandrel 72 for directing drilling fluid flow to the drill bit.
Looking at the arrangement for this fluid flow bore 93 of the bottom driveline 90 in more detail, the top end of the second transmission shaft 92a is coupled to the piston mandrel 92b with a universal joint 94b, and the bottom end of the second transmission shaft 92a is coupled to the bearing mandrel 72 with a universal joint 94a. This second transmission shaft 92a allows the motor housing to be bent to facilitate directional drilling. Seal boots are not necessary here at the joints 94a-b because the bottom driveline 90 is contained in a sealed oil chamber 67.
To prevent drilling fluid from entering the oil chamber 67 via the central bore 93, seal journals 96a-b are threaded into each drive adapter of the joints 94a-b with an O-ring to seal the threads. Each end of the drive shaft bore 93 inserts onto the journals 96a-b with an internal O-ring to create a seal. The journals 96a-b remain fixed to the adaptors for the joints 94a-b, while the second transmission shaft 92a can articulate to an extent. The seals between the shaft's bore 93 and the journals 96a-b are located at a center of rotation of the joints 94a-b to reduce the geometrical changes at the sealing site. The ends of the shaft's bore 93 are also machined at certain angles to allow the joints 94a-b to articulate a small amount when the motor 60 is bent so the second transmission shaft 92a can avoid contacting the journals 96a-b. 
The fixed journals 96a-b for the joints 94a-b are suited for sealing fluid passage to the drill bit because the transmission section has two transmission shafts 82 and 92a to reduce the amount of articulation at each joint 84a-b and 94a-b. As shown in FIG. 3D, for example, the motor 60 is shown with a 2-degree bend in which the two transmission shafts 82 and 92a compensate for eccentricity in the power section and for bend in the housing. In particular, the joints 84a-b of the first transmission shaft 82 compensate for the eccentricity of the power section (given here as angles Γ and Ω of 0.58-degrees). The joints 94a-b of the second transmission shaft 92a compensate for the bend in the housing (given here as angles β of 0.80-degrees and α of 1.20-degrees). At these lower bend angles, the fixed journals 96a-b inside the second transmission shaft 92a can seal close to the center of rotation of the joints 94a-b so the sealing profile will change the least as the joints 94a-b articulate.
As can be seen above, a bore in a shaft of a prior art mud motor can be conventionally used to convey drilling fluids to a drill bit as in the arrangement of FIGS. 3A-3D. Alternatively, a bore in a shaft of a prior art mud motor can be used for passage of wires, as in the arrangement of FIG. 2. However, arranging a motor to achieve either one of these purposes of ported or wired communication through a shaft while transferring motor motion to rotational motion and still allowing for bending during use requires a mud motor to be considerably longer and more complex than desired for downhole operations.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.